This invention relates to a method and apparatus for forming fiber-reinforced thermoset plastic articles. In particular, the invention relates to a technique for forming such articles by a multiple shot technique wherein the injection ram does not traverse the stuffer passage in the injection cylinder at the end of each injection shot into the mold to thereby avoid degradation of the fiber reinforcement by cutting or balling as the injection ram shears the uncured resin mixture transferred from the stuffer unit to the injection cylinder.
This invention further relates to an injection molding technique which includes the step of precompacting the fiber/resin mixture to express gases therefrom prior to injecting sequential portions of the precompacted mixture into the mold cavity.
Various techniques have been employed to mold fiber-reinforced thermoset plastic articles. Those techniques include bag molding, centrifugal molding, hand lay-up, and sheet molding. Injection molding of such articles has been a fairly recent development as compared to the injection molding of thermoplastic plastic articles due to the process complexities which add to the cost of the finished articles. Injection molding techniques, however, produce articles having complex curvatures and details not possible by other molding techniques such as sheet molding techniques.
The injection molding of fiber-reinforced thermosetting resin is far more complicated than the injection molding of thermoplastic resins. One major distinction between the two is the fluidity characteristics of the resin systems. Liquid thermoplastic resins have a much lower viscosity than fiber-reinforced thermosetting resins. Thermoplastic resins generally have the fluidity characteristics of syrup, while fiber-reinforced thermosetting resins have the viscosity of modeling clay. While some thermosetting resins which are capable of being injection-molded have relatively low viscosities, those resins are not suited for applications involving a fiber-reinforced compound, since the low viscosity of the resin is incapable of carrying the fibrous reinforcement throughout the mold cavity in a uniform fashion if, indeed, they are capable of conveying the fibers through the injection nozzle.
In addition to the disparity in viscosities, the setting characteristics of the two resin systems are such that the thermoplastic resins need only be heated in the injection cylinder and then solidified by cooling in the mold. Thermosetting resins, on the other hand, must be carefully maintained at a predetermined temperature in the injection cylinder which is high enough to permit the injection of the reinforced plastic into the mold but not high enough to achieve cross-linking of the polymer. Molding pressures must also be closely controlled since they influence molding temperatures.
Fiber-reinforced thermosetting resins for injection molding purposes are usually employed in the form of bulk molding compounds (BMC) having the consistency of modeling clay. Bulk molding compounds offer simplified handling for high volume compression molders. BMC is a mixture of short (1/8 to 11/4 inch) glass fibers and resin containing filler, catalyst, pigment, and other additives as required by the end use application. The material is premixed and can be provided in bulk form or can be extruded into rope or log-like forms for easier handling by high volume injection molders. Sheet molding compound (SMC) or thick molded compound (TMC) may also be employed.
Whether the bulk molding compound is injection-molded by a ram or by a screw technique, there are inherent complications which add to the difficulties involved in the injection molding of fiber-reinforced thermosetting resins. As is described in U.S. Pat. No. 4,280,972, the shear forces developed by a conventional plasticizing screw, or even a deeper flighted conveying screw, tend to break the glass fibers. This results in lower product strength because the short, broken fibers are not as effective a reinforcing constituent as are longer fibers. In plunger or ram-type injection molding, the bulk molding composition is fed to the barrel of the injection molding machine by a feed or stuffer tube generally transverse to the axis of the injection barrel. Conventional molding techniques involve the retraction of the injection ram rearwardly of the feed tube opening, forcing a metered amount of molding compound from the feed or stuffer tube into the molding cylinder and driving the injection ram through its cylinder to inject the molding compound into the mold. This procedure is repeated for each injection shot and tends to break or shear the fibers at the intersection of the feed tube and the injection cylinder during each cycle. In addition to severing the fibers, it has been found that this action tends to form fibrous balls.
Prior art injection molding techniques further rely upon the travel of the injection plunger as a determination of the correct volume of resin injected into the mold. In practice, however, the resin and fiber mixture may not be fully compacted, but may contain pockets of air and/or other gases which are trapped during the fiber-resin blending operation.